The correct answer is money.
It's rather "power", in various forms. Women want a dominant attribute that they can admire and rely on. For some it's money, for some it's physical strength, for some it's intelligence, for some the amount of friends he's got, for some it's humor, for some it's sex skills, etc...
A man needs to have SOMETHING to offer.
The claim "money" is not insulting, it's just a tad too simple. A LOT of women look for wealthy men. Or wealthy women, if they are homosexual. Money means you're successful.
And always remember, it's the diameter, not the length.